OK I've got the answer from a very helpful "James" from James Shirlock BMW Motorcycle Spares:
The R1100S floating stud bolt is - M24 x 1.25
That's good news because a couple of weeks ago I payed out £65 for an M24 x 1.25 tap to chase the thread in my swinging arm. I slightly damaged the last couple of threads when removing the stud bolt. I understand that this sort of oops is not that uncommon, and after chasing the thread I was very pleased with the result
, until......
My friend John asked to borrow the tap (for his next door neighbour) to chase the thread on an older R1100RT they were working on. Day later John returns with the tap and the RT stud bolt, looking very pissed off. John's neighbour had started 'chasing' the thread until John noticed that he was cutting a totally new thread
And sure enough the RT stud bolt is a coarser pitch thread, it's shorter and the locking nut has no flange.
At this point I seriously started to doubt my ability to measure a thread
(ex-toolmaker) as if, but I started a new topic just in case.
So I think there is a lesson here for us all:
Don't lend any of your tools to my mate John, as his next door neighbour is a total f@!*ing idiot!